Judging from your other posts, it seems that you're having trouble understanding and accepting your partner's crossdressing. Have you talked with him much about it? Sincerely and non-judgmentally asked your questions?

Googling will turn up a lot of information on this subject. A lot of men who cross-dress don't even know why they like to do it. Some men just like feel of lingerie. Some men want to get in touch with a feminine side. Some men may have gone from being aroused by looking at women in lingerie to fetishizing the lingerie itself. Some submissive men enjoy being humiliated (forced cross-dressing being a practice in this case). The list goes on and on.